I have both wood and plastic boards, but find myself using the plastic more often. I use lemons for a lot of things, so always have those handy, and white vinegar. Next time you use your board (plastic or wood) take half a lemon and capful of vinegar and rub...kinda like you are polishing. Do this as soon as you are finished, do not let food stains sit for any amount of time on your boards. Vinegar and lemons are natural bleach (not harmful if ingested) they are also antibacterial. You can choose to wash with soap and water afterwards, but this is all I do, and I give it a good rinse.
As far as lettuce, only wash what you are immediately using. I have never cleaned an entire head to let it sit in the fridge for future use. Moisture is not your friend in this case.
HTH